The present invention relates to a piston for an axial piston machine. The piston can be a piston for an axial piston machine with a swash plate design or a wobble plate design.
In known axial piston machines solid pistons, which set limits on an operation with higher speeds, are used. At increased speeds, strength problems arise for the cylinders due to large centrifugal forces, and due to the large mass forces strength, problems arise for the piston retaining means. Furthermore, thermal problems can arise at the contact surfaces between piston and cylinders, as high frictional forces result from the centrifugal forces.
As an alternative to solid pistons, hollow pistons therefore are used, in order to operate axial piston machines with increased speed. In usual hollow pistons the hollow piston body includes a cavity open towards the working cylinder. However, this has the disadvantage that the cavity forms a dead space which is compressed and then decompressed again at each piston stroke. The non-negligible compressibility of the hydraulic fluid leads to a deterioration of the efficiency.
It is known from various documents, such as U.S. Pat. No. 3,319,575, German Patent application document DE 199 34 217 A1, and German Patent application document DE 2 364 725 A1, to close the cavity provided in the piston body in a pressure-tight way. As each cavity, an annular space is provided, which extends between an outer cylindrical sleeve and a central column. On their end faces, facing the cylinder space, the cylindrical outer sleeve and the central column are provided with an end closure which closes the annular cavity towards the working side of the piston. In the central column an axial pressure lead-through extends, which serves the lubrication of the slipper of the piston.
Such a piston design however, can lead to problems during friction welding, which is usually employed for mounting the closure, since the central column is greatly loaded. In addition, the manufacture of the annular cavity by extrusion is expensive. From German Patent application document DE 36 02 651 A1, it is known to design the cavity provided for reducing the mass hollow-cylindrical as a whole, and to let the fluid supply channel for the lubricating oil supply extend in the piston skirt wall. However, this in turn requires an expensive design of the piston wall and in addition leads to stability problems with the piston.
From German Patent document DE 197 06 075 C2, it is known to fabricate the piston of an outer tube and an inner tube, each of which, as raw parts, is hollow and cylindrical, with the blank for the outer tube resting against the inner tube by deformation in its end regions, so that an annular space in turn is formed in the piston body. Furthermore, the ball for a ball joint connection with a slipper also will be fabricated by deforming the blank for the outer tube, whereby a ring-shaped hollow space also is obtained in the ball head. There also exists a complicated manufacturing method for the piston, without sufficient stability of the piston being ensured.